Can someone please tell me how to find the center of gravity of the front of my car so that I can find the angle of my effective moment arm ?? Also doesn't the more horizontal the EM arm give you more vertical load on the RF than a more vertical EM arm ?? Thanks for info !!!
Can someone please tell me how to find the center of gravity of the front of my car so that I can find the angle of my effective moment arm ?? Also doesn't the more horizontal the EM arm give you more vertical load on the RF than a more vertical EM arm ?? Thanks for info !!!
Are you talking about the "Roll Center" or "Center of gravity"? I only ask because COG is usually relevant to the whole car where the RC is either front or rear...
Left 4M and Dirt Late Model racing, 04/12/2016 @12:06AM.
Are you talking about the "Roll Center" or "Center of gravity"? I only ask because COG is usually relevant to the whole car where the RC is either front or rear...
I got a RC program I need to find the height & side to side of the COG on the front so that I can find the distance between them to find the length & angle of my front moment arm !!
To find the front or rear COG averages, you can do just like the regular COG but this way and you have to do it twice to get a rough estimate on the differences between the front and the rear.
Again been a while and I'm getting old:
Do the same as normal for calculating COG but:
1. Find the rear weight of car in % say 54.0% rear weight
2. measure wheel base say 104", then find the location of the overall COG vs wheel base
3. 104" x .54 = 56.16" back from front tires.
4 Do the same thing but instead of blocking raising the car and pivoting off the other set of tires, you need to pivot the car at the 56.16" back from front tires
5. This will give you and average of the front vs the rear, you need to set the pivot as the same height of the other axle or center of spindles. Again you have to do it twice once raising the back and then again raising the front but using the same pivot point.
You are basically dividing the car in half at that 56.16" behind the front tires. This way you are finding the COG of middle of the front half of the car and the COG of the middle of the back of the car.
This will give you the COG of roughly half way between the pivot and the front/rear tires. With these 2 COG's you can basically draw a line between the 2 and extend out to the actual wheel center lines and get a rough idea where each is. You can double check yourself as the pivot point's COG should be the same as finding the normal COG and if it's off then you did something wrong.
Maybe that makes sense to ya and that should lead you in the right direction but may have some mistakes as I said
for what i THINK you might be trying to accomplish you need to determine what your front and rear roll centers are. the imaginary longitudinal line that connects the two is your roll axis (moment center). you can use your scale % to easily get a CG location, then an appropriate formula and scales to determine VCG (verticle center of gravity). the imaginary line drawn perpendicular to and up (presumably) from the roll axis up through the VCG is your moment arm.
you are trying to determine VCG and its relationship with the roll axis?
Basically I'm dividing the car in 2 half's and finding the COG for each half. You will find the COG for the center of the back half of the car and the center of the front half of car.
Now you have those 2 points you draw a line between the 2 to extend that line out to the center of each axle/tire centerline, thus giving you the approximate COG of the car for the front and the back at the tire centerlines.
You are basically adding an axle at the middle of the car and doing the front half of the car then the back half.
Maybe this drawing will help make more sense. The black circles are the COG's you are going to find by dividing the car in half by using a pivot point method. Then drawing the line between the 2 black COG's and extending to the wheel centerlines will show the approximate COG at the tires for the front or the back.
I should add, the part I can't remember is the overhang weight when doing the car in halves. That was the reason I used the rear weight vs the wheel base so I knew the overhang weight was half the car weight. But for the life of me I can't remember what I had to do to counteract this and make it come out right.
Old age is catching up and too lazy to sit down and figure it out again, lol
I should add, the part I can't remember is the overhang weight when doing the car in halves. That was the reason I used the rear weight vs the wheel base so I knew the overhang weight was half the car weight. But for the life of me I can't remember what I had to do to counteract this and make it come out right.
Old age is catching up and too lazy to sit down and figure it out again, lol
the part that you are forgetting is the "overhang weight" is what is used to determine VCG or Verticle Center of Gravity. THAT is the important number. CG is only relative to your % front to rear, side to side. VCG is dynamic in that there is an element of weight transfer as the car goes around the track. In other words, i don't care what my CG is - my % left and rear tell me that. I DO care how high or low weight is located in the car relative to my roll center.
hopefully the poster can elaborate a bit on what he is really trying to determine....
Thanks guys I am using the race wise software ! I don't have to have the COG to find my RC but what I am wanting to do is find the difference in my RC height and my COG height !! I would like to know my moment arm length& angle !! Also is there a way to find the side to side where the COG is ?? I may be completely over thinking this here but would the angle. Of the MA not make a huge difference in load in the RF horizontally or vertically ?
I am thinking like a pry bar here if it is vertical & you pull on it it's going to slide across the floor but I'd you pull down more vertical it will pull it into the ground !! ?? Reply or pm me with ideas it may be a easier way than what I am trying to do !! Thanks
I was figuring Vertical COG as when I talk about COG I talk all three axis, but since no one is following me so I'll quit trying as it was a pain in the arse to figure it out the first time and it's been years since I tried it again.
dirtdiggerracing: Left and rear % will tell you where the COG is located left/right and front back but not vertically. If you want the hieght you have to follow the procedure of the link that DaveBauerSS6 posted. However this is for the entire car not just front or back, which can be a reasonable amount of difference, but for the most part isn't necessary. I had my reasons for figuring out the differences between the two.
what you are wanting is the link Dave posted to find hieght, then figure the left/right by your left side % and you will have the COG locations needed to see the momentum arm.
Sorry I over complicated what you where looking for, I thought you wanted the specific height of the front end only for a certain reason. However what your looking for, it's overkill and not necessary just use the cars overall COG hieght.
I really don't follow on this two COG thing. When talking applied forces, there is only one COG. That COG can apply force in only your tire contact patches.
When applying force to the side, there are two roll centers that determine how much side or down force is applied to each side tire, but that force always comes from THE COG. This also happens in front to rear forces. Instead of roll centers, you have anti squat values(4 bar settings) and anti dive (angles of control arms) for front. If you asymmetrical settings, Different side to side and front to rear, you can juggle how much weight and in what direction it is applied.
Think of it as a man pushing a refrigerator. Push too high the fridge will topple over away from the push. Too low and it will topple over on the man pushing. Think of where the man pushes, as your roll centers or anti squat values. These values or roll centers relationship to the COG of the fridge, determine how much down force is at his two feet (tires).
It is the relationship of the COG and your roll centers and bar settings that determine how much force is applied downward ( side bite / forward bite) or sideways at any tire (lack of bite).
Wow...
There is absolutely no such thing as TWO centers of gravity in a car...unless the car is physically articulated (which it is not).
The reason there are two roll centers (front and rear) is because the front and rear suspensions are completely independent of one another. But there is only ONE chassis and ONE mass that is being sprung so to say that there are somehow two centers of gravity is completely incorrect. The ENTIRE mass of the car acts on the front suspension and the ENTIRE mass of the car acts on the rear suspension. There are not separate masses therefore there are not separate centers of gravity.
I'm always open to new and fun ideas with vehicle dynamics but we do need to work within the laws of physics.
I sent a link on the first or second day of this thread to the guy asking on how to figure COG by PM. Last time I saw the COG at work was when two wreckers picked up a wrecked mod at both ends and it flipped over, guess they didn't find it. Past that I will sit in my corner and say nothing except to say what Matt said, WOW.
Every oz of weight from the bolt fastening the radiator in, to the decal on the spoiler all adds to the center of gravity.
When you talk about overhang weight, I believe you are talking about polar moment. Weight closer or farther away from the COG. This weight still is part of the Mass of the car but can change the rate of transfer when applying forces.
Think of the merry go round at the park. If your way out on it, it takes effort to stay on. If your in the center, you almost need no hands to stay there. These are the forces at work with polar moments. High polar moments can turn your car into a bicycle or low polar moment can rob you of side bite. Too much rear can spin you out and not enough can make it hard to turn. What you want is a compromise that allows your roll centers, anti squat and anti dive to become tools to fine tune the force.
I know putting bar A in hole C is the way we fine tune chassis now days, but I think it adds something, if you know why. It's all based on physical law and knowing a few of those laws helps your confidence on choices of bar A in hole C. or D.
Senior Member
Center of gravity
RC or COG?
Are you talking about the "Roll Center" or "Center of gravity"? I only ask because COG is usually relevant to the whole car where the RC is either front or rear...
Originally Posted by dirtdiggerracing
Left 4M and Dirt Late Model racing, 04/12/2016 @12:06AM.
Reply With Quote
Posting Permissions
Bookmarks